The slag residues from crucible OB 345 display a different microstructure: they have a glassy slag notably enriched in titanium oxide and calcium oxide (Table 5.4), and inclusions of residual quartz, titanite (CaTiSiO5) and fluorine apatite
(Ca5(PO4)3F) (Fig. 5.25). This specific slag composition suggests the processing of a
mineral different from the one identified above for most of the slag remains. This source could have been an ore whose gangue was made of titanite, apatite, and quartz, rather than quartz and calcite as suggested from the results of the main set of samples. The various residual minerals of this slag, only partly dissolved in the glass, are unlikely to come from the erosion of the ceramic. Titanite is non-existent in the ceramic fabric and apatite is too rarely found to contribute on its own to the large number of crystals of this mineral in the slag. Doubts only remain on the quartz inclusions, since quartz grains constitute the most abundant inclusion in the ceramic
fabrics, making up to a quarter of the total volume (Martinón-Torres 2005: 111; Martinón-Torres and Rehren 2005b).
Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 FeO CuO ZnO Sb2O3PbO SO2 wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% Glass matrix of slag layer 3.9 2.0 13.4 55.2 0.9 3.3 8.4 5.7 6.5 b.d.l. 0.1 b.d.l. b.d.l. 0.6 Bulk of matte residue b.d.l. 1.1 1.6 12.4 b.d.l. b.d.l. 5.2 1.6 24.4 b.d.l. b.d.l. 24.0 21.8 7.9 Main phase of matte residue 0.3 0.5 0.8 6.7 b.d.l. b.d.l. 3.8 0.8 6.4 0.7 b.d.l. 38.4 41.6 b.d.l.
Table 5.4. Average chemical composition by SEM-EDS, normalised to 100 wt%, of
the slag layer and oxidised matte residues of crucible OB 345 (b.d.l.: below detection limit). See appendix 2 p.312 for the full results on the glassy matrix of the slag layer.
Fig. 5.25. Slag composed of a grey
glass and residual crystals of quartz (dark grey) and titanite (light grey). Note the various shades of grey around the remaining minerals, indicating they are gradually dissolved in the glass matrix (OB 345/S1, BSE, 200x).
Many crystals of magnetite can also been identified, close to the top surface of this slag. These display bright lamellae of haematite, indicating that close to the rim, where this sample is from, the conditions were oxidising. The charge, however, was probably melted under reducing conditions, as implied by the dendrites of pyrrhotite, occurring in the corroded matte remains of this sample. The significant iron content of this oxidised layer of matte (24 wt% FeO; Table 5.4) is visible in the presence of these numerous dendrites of pyrrhotite, some of which have oxidised in the fire to form the magnetite crystals previously mentioned (Fig. 5.26). The latter have developed in the liquefied charge thanks to the reaction of pyrite and its burnt
oxygen available from the forming slag. That is why the magnetite crystals can now generally be seen at the interface of this oxidised matte with the slag.
Fig. 5.26. Large dark grey crystals
of magnetite in a bright matrix rich in the oxides of lead and antimony (OB 345/S1, BSE, 1000x).
The oxidising melting of a lead bullion
The bulk of crucible residue OB 472/S1 is very rich in lead and antimony oxides (44 wt% PbO, 38 wt% Sb2O3), with relatively high levels of silica (9 wt% SiO2) and
alumina (5 wt% Al2O3), in addition to a little potash (2 wt% K2O), calcium oxide (1
wt% CaO) and soda (1 wt% Na2O) (Table 5.5). The bulk sulphur concentration of
this sample is too low to be detected; sulphur is only identified in the numerous crystals of leucite and lazurite-like feldspathoids separate from the main matrix of lead and antimony oxides (50 wt% PbO, 43 wt% Sb2O3) (Fig. 5.27); furthermore no
copper or iron were detected in this sample, contrary to the matte residues described above.
Na2O Al2O3 SiO2 K2O CaO Sb2O3 PbO
wt% wt% wt% wt% wt% wt% wt%
Bulk 0.7 5.1 9.2 2.0 1.5 37.8 43.7
Main phase b.d.l. 1.5 5.2 b.d.l. b.d.l. 43.1 50.2
Table 5.5. Average chemical composition by SEM-EDS, normalised to 100 wt%, of
Fig. 5.27. Numerous dark grey
crystals similar to leucite and lazurite in a bright matrix rich in the oxides of lead and antimony (OB 472/S1, BSE, 200x).
The overall richness in lead and antimony, the low sulphur content, and the absence of copper and iron suggest that the residue from crucible OB 472 is almost certainly an oxidised bullion. This is further verified by the analysis – presented below – of the matte cakes, which are relatively rich in iron, and the isolated lead bullion find, whose bulk is mainly composed of lead and antimony. The unusual number of feldspathoid crystals forming in a relatively silica-poor environment points towards the use of alkali-rich fluxes, since a metallic bullion on its own could not be the source for these specific elements in such a significant concentration. This again suggests that potassium, sodium, calcium and silica were intentionally added, with part of the silica and alumina probably coming from the ceramic. The high levels of these elements in these crucible slag remains strengthen the hypothesis that glass and salt were added to the charge, and are therefore probably responsible for most of this enrichment. The presence of the leucite and lazurite-like crystals in the oxidic matrix also indicates that the oxidation of lead and antimony is not post- depositional but took place under high temperature. The microstructure and elemental composition of this sample seem to indicate a different operation: this procedure could have been the oxidising melting of a bullion with glass and salt, this bullion having been produced in a triangular crucible beforehand through the main reaction, characterised above, of the reducing fusion of a sulphidic ore.
This procedure may have been the cleaning operation of a bullion to remove the residual matte, which may still be attached to it or contaminating it in the form of inclusions. Such a procedure would however produce an iron oxide-rich slag, which
laboratory, the present sample may reflect a final and possibly unnecessary refining of an already clean bullion, which would be devoid of matte and therefore not contribute iron oxide to the slag. There is however no real possibility to confirm the intention of the application of this process in light of the present data and one can only conjecture such a purpose.
The use of antimony as a collector for noble metals
The crucible OB 479 has some remaining mixture of matte and metal, which is still attached to its slag. The bulk of the metallic residue is composed of copper (50 wt% Cu) and antimony (13 wt% Sb), now corroded to oxides (19 wt% O) and chlorides (15 wt% Cl). Copper forms the main matrix (56 wt% Cu, 25 wt% O, 19 wt% Cl), in which separate antimony-rich phases can be seen (57 wt% Sb, 19 wt% Cu, 14 wt% O, 10 wt% Cl) (Fig. 5.28). In this metallic environment, some corroded and round matte inclusions of copper sulphide can also be noticed (70 wt% Cu, 16 wt% S, 9 wt% Cl, 5 wt% O).
Fig. 5.28. Metallic residue composed
of a copper-rich matrix and bright antimony-rich phases (OB 479/S1, BSE, 250x).
The slag layer in this crucible has a particularly high concentration of antimony oxide (41 wt% Sb2O3), whilst it is completely devoid of lead (Table 5.1, p. 121).
This might be an indication of an even wider range of materials used at Oberstockstall, perhaps including antimony sulphide, which could have been used as a flux, if following one of Ercker’s recipes (Sisco and Smith 1951: 114). This residue may also indicate an attempt to use antimony as a collecting agent for the
appearance and properties between both metals. It is important to emphasise that, in the sixteenth century, the nature and properties of antimony were not understood. Antimony is mentioned by Biringuccio (Smith and Gnudi 1990: 91-92) and Agricola (Hoover and Hoover 1950: 400, 428), but they do not recognise it as a metal and usually use the same word – stibium – for the metal itself and its sulphide. Ercker does not acknowledge it as a metal either: “[a]lthough there are other additional mineral ores that can be liquefied and smelted, such as bismuth, sulphur, antimony, and so forth, the aforesaid seven [metals: gold, silver, mercury, copper, iron, tin, and lead] head all the other minerals as the most important ones.” (Sisco and Smith 1951: 4, my italics). This specific matter of awareness and understanding of the various materials and operations by the individuals using the laboratory of Oberstockstall will be discussed more broadly in a later section (cf. chapter 7).
In the sixteenth century, stibnite was in metallurgy mainly used for the parting of noble metals (Hoover and Hoover 1950: 451-452; Smith and Gnudi 1990: 201-202). The principle of this method is based on the greater affinity of silver for sulphur than antimony. Antimony plays the role of sulphur donor and gold collector, while silver preferentially reacts with sulphur. Two separate phases of silver sulphide and a gold- antimony alloy form in the hot crucible, with the metallic phase settling at the bottom. The corroded metallic residue of crucible OB 479, however, does not appear to illustrate this operation, since it contains too much copper in it, which would otherwise have been removed from the gold-silver bead through cupellation.
This crucible also presents another particularity: a relatively homogeneous layer in between the ceramic and the slag itself, which is highly enriched in potash (13 wt% K2O) compared to its two surrounding phases (3 wt% K2O in ceramic bulk, 7
wt% K2O in glass matrix of slag). This layer could have been deliberately added by
the potter to the ceramic: a feldspar lining may have been applied to the crucible wall possibly in order to protect the vessel itself against corrosion or attack by the reagents used in metallurgical operations, similarly to glaze layers in stoneware (Gaimster 1997). This would seem however rather strange, knowing that all the graphitic crucibles bear the same stamp and therefore come from the same ceramic workshop. It would appear unusual to have one unique crucible having this specific layer, while the others do not and still perform their function well, withstanding the heat and the attacks from chemical reagents. Besides, the feldspar lining would
suggesting further that the presence of such a lining is rather unlikely. Another explanation would be the addition in this case of a great quantity of saltpetre (potassium nitrate), instead of a sodium-rich salt, used as a cover for the charge. This potassium nitrate may have reacted with the hot ceramic wall, forming this glazing on the surface of the crucible wall.